Bismaleimides are being increasingly used as matrix resins for fiber-reinforced composites, especially high-performance graphite fiber-reinforced composites. The monomers are cured by a thermally induced addition reaction to give highly crosslinked, void-free network polymers having good physical properties with higher thermal stability, higher char yield, better fire resistance and lower water absorption than currently-used epoxy systems.
There are problems with maleimides, however, such as solvent retention in the prepregs, high temperatures often needed for curing with attendant distortion and high cost, and the brittleness of the polymers due to the high crosslink density obtained in network polymers. Brittleness can be such a problem that a single minor impact can greatly reduce the strength of a maleimide-based composite. These problems have prevented the wide application of the carbon-reinforced maleimides in aircraft primary and secondary structures where their high strength and fire resistance is much sought after.
A number of methods have been proposed to minimize some of the aforesaid problems. For one, the basic maleimide structure can be modified. For another, the bismaleimides can be copolymerized with comonomers. The present invention concerns vinyl pyridine group-containing compounds and their incorporation as comonomers into bismaleimide resins. One of the types of vinyl pyridine group-containing compounds has a vinyl styrylpyridine structure, i.e., a ##STR1## structure wherein n is 1 to 10, each of the R.sub.1 is hydrogen or lower alkyl and R.sub.2 is an organic group. U.S. Pat. No. 4,362,860 of Ratto et al discloses such materials as well as the materials where R.sub.2 is hydrogen as thermosetting polymers but does not suggest their copolymerization with bismaleimide. Rockwell International's Final Report (Phase I) (Aug. 18, 1980 -Nov. 30, 1981) in NASA contract NAS2-10709 further discusses the Ratto et al findings where R.sub.2 is hydrogen and shows that the one step synthesis route it discloses gives a mixture product which must be fractionated to obtain the desired vinyl styrylpyridine materials.
U.S. Pat. Nos. 3,810,848 and '872 disclose a family of complex-forming polymers having ##STR2## units, wherein L is a covalent bond or O, S, CO, CHR or NR and R is hydrogen or an alkyl. At Example 6, U.S. Pat. No. 3,180,848 shows condensing such a polymer with a pyromellitic anhydride to give a maleimide-like copolymer structure. Another patent of interest is U.S. Pat. No. 3,994,862 of Ropars, et al. which discloses condensation products of trimethylpyridine with aromatic aldehydes to give prepolymers which differ from the present vinyl-terminated styrylpyridines in not having the vinyl terminis. Ropars, et al. also does not show copolymers with bismaleimides.